1. Field of the Invention
The present invention generally relates to a novel family of thyristors and, more particularly, to a family of low cost metal oxide semiconductor (MOS) emitter turn-off thyristors.
2. Description of the Prior Art
Thyristors, sometimes referred to as silicon controlled rectifiers, are four layer pnpn devices comprising an anode, a cathode, and a gate terminal. Thyristors are designed to carry very high currents with very little voltage drop. For example, at currents of up to 500 A, the voltage drop across the anode and cathode terminals typically does not exceed 2 V. This makes thyristors ideal for power electronics switching applications such as converting one power form to another, such as, for example dc-ac or dc-dc.
Standard thyristors are turned on by applying a short current pulse across the cathode and gate terminals. Once the device is turned on, high currents may flow between the anode and cathode. Unfortunately, the gate can only be used to turn the device on, it cannot be used to turn the device off. Turn-off is accomplished by applying a reverse voltage across the anode and the cathode. Several variations of the standard thyristor have been developed to facilitate the turn-off operation such as, for example, gate turn-off thyristors (GTO) and metal-oxide semiconductor (MOS)-controlled thyristors (MCTs).
Gate turn-off thyristors (GTO) have been developed which can turn the device off by applying a reverse gate pulse to bypass carriers directly to the gate circuit. However, GTO's are known to have a poor turn off current gain. For example, a GTO having a 2000 A peak current may require up to 500 A of reverse gate current.
The MOS-controlled thyristor (MCT) has been around for about a decade and is basically a thyristor including two built in MOS transistors, one to turn the thyristor on, and one to turn it off. The gates of the two MOS transistors are tied together. A relatively low negative voltage pulse (i.e., −7 V) to the gates turn the thyristor on, and a positive pulse (i.e., 14 V) to the gates turn the thyristor off. Hence, the MCT has the advantage of being able to be driven directly by logic gates. However, MCTs are typically expensive to fabricate and very difficult to scale to high voltage (e.g., >2 KV) and high current (e.g., >100 A).